1. Technical Field
The present invention relates to methods for manufacturing ferroelectric memories.
2. Related Art
Ferroelectric memories (FeRAM) are nonvolatile memories capable of low voltage and high-speed operation, using spontaneous polarization of ferroelectric material, and their memory cells can be each formed from one transistor and one capacitor (1T/1C). Accordingly, ferroelectric memories can achieve integration at the same level of that of DRAM, and are therefore expected as large-capacity nonvolatile memories. As the ferroelectric material, perovskite type oxides such as lead zirconate titanate (Pb (Zi, Ti) O3: PZT), and bismuth layered compounds such as strontium bismuth tantalate (SrBi2TaO9: SBT) can be used.
In order to make the aforementioned ferroelectric material exhibit its maximum ferroelectric characteristic, its crystal orientation is extremely important. For example, when PZT is used as the ferroelectric material, a predominant orientation exists depending on its crystal system. Generally, when PZT is used in memory devices, titanium-rich compositions that contain a greater amount of Ti (titanium) compared to Zr (zirconium) is used in order to obtain greater spontaneous polarization. In such a composition range, PZT belongs to a tetragonal system, and its spontaneous polarization axis aligns with the c-axis. In this case, ideally, the maximum polarization can be obtained by orienting it in the c-axis, which is in effect very difficult, and a-axis orientation components perpendicular to the c-axis concurrently exist. It is noted that because the a-axis orientation components do not contribute to polarization inversion, the ferroelectric characteristic may be impaired.
In this respect, it has been conceived to orient the a-axis in a direction offset at a predetermined angle from the substrate normal line by making the crystal orientation of PZT to a (111) orientation. As a result, the polarization axis has a component in the substrate normal line direction, and thus can contribute to polarization inversion. On the other hand, the c-axis orientation component concurrently has its polarization axis oriented to a predetermined offset angle with respect to the substrate normal line direction, such that a certain amount of loss occurs in the amount of surface charge induced by polarization inversion. However, the entire crystal components can be made to contribute to polarization inversion, such that the charge retrieving efficiency significantly excels, compared to the case of the c-axis orientation.
As a method for forming a ferroelectric film with its PZT crystal orientation aligned to a (111) orientation, a method described in Japanese Laid-open Patent Application JP-A-2003-324101 may be used. According to the method described in the aforementioned document, a lower electrode with a (111) crystal orientation is formed from iridium, a surface layer on its upper surface side is thermally oxidized to form an iridium oxide layer, and then a ferroelectric film is formed on the iridium oxide layer. At the time of forming the ferroelectric film, a MOCVD method is used, in which source material gas for the ferroelectric film and oxygen gas are chemically reacted for forming the film. According to this method, the film formation is conducted with a smaller amount of oxygen gas than the amount of oxygen gas necessary for the chemical reaction, and then the film formation is further conducted with an amount of oxygen gas greater than the amount of oxygen gas necessary for the chemical reaction. Although details of the mechanism thereof are not clarified, the iridium oxide layer contributes to determination of growth orientation of PZT, and makes PZT mainly orient to a (111) crystal orientation.
By the method described in the aforementioned document, the crystal orientation of PZT may be improved, but the method entails some points to be improved in order to improve the characteristics of the ferroelectric film. Specifically, iridium that serves as a base for iridium oxide has a very high melting point, and therefore, when a film of iridium whose crystal orientation is in a (111) orientation is formed, the film has a polycrystalline structure having plural crystal grains in a (111) orientation arranged in parallel. Then, when a surface layer thereof is oxidized by thermal oxidation or the like, oxygen gas would more likely penetrate boundary portions of the crystal grains such that these portions would be abnormally oxidized, and protrusions caused by volume expansions due to the abnormal oxidation are created in the iridium oxide layer at positions corresponding to the crystal grain boundaries.
When growing PZT, such protrusions would likely become starting points of the crystal growth, and PZT would often grow in protrusions on them. Protrusions of PZT greater than the protrusions of iridium oxide may at times be generated in the ferroelectric film. PZT formed on the protrusions in the iridium oxide layer do not have a (111) crystal orientation, and therefore its contribution to polarization inversion is reduced, which presents an obstacle to improvements of the characteristics of the ferroelectric film.